Production and Fabrication
Several steps are involved in the extraction of tantalum from tantalite. First, the mineral is crushed and concentrated by gravity separation. This is generally carried out near the mine site. Further processing by chemical separation is usually done by treating the ores with a mixture of hydrofluoric acid and sulfuric acid at over 90 °C. These reagents dissolve the tantalum and niobium oxides to give complex fluorides, which can be separated from impurities and from each other:
- Ta2O5 + 14 HF → 2 H2 + 5 H2O
- Nb2O5 + 10 HF → 2 H2 + 3 H2O
The first industrial-scale separation, developed by de Marignac, used the difference in solubility between the complex niobium and tantalum fluorides K2•H2O (potassium oxypentafluoroniobate monohydrate) and K2 (potassium heptafluorotantalate) in water. Newer processes use the liquid extraction of the fluorides from aqueous solution by organic solvents such as cyclohexanone. The complex niobium and tantalum fluorides are extracted separately from the organic solvent with water, and either precipitated by the addition of potassium fluoride to produce a potassium fluoride complex, or precipitated with ammonia as the pentoxide:
- H2 + 2 KF → K2↓ + 2 HF
- 2 H2 + 14 NH4OH → Ta2O5↓ + 14 NH4F + 9 H2O
The purified K2 is generally treated by reduction with molten sodium to produce a coarse tantalum powder.
In terms of fabrication, all welding of tantalum must be done in an inert atmosphere of argon or helium in order to shield it from contamination with atmospheric gases. Tantalum is not solderable. Grinding tantalum is difficult, especially so for annealed tantalum. In the annealed condition, tantalum is extremely ductile and can be readily formed as metal sheets.
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